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I see a similar question answered, but this is perhaps more specific. There's a lot of description about the James Web Space Telescope (JWST) operation during its planned mission. But I gather JWST has propellent only for a decade or so, at which point it will no longer be able to maintain the L2 Halo orbit. However, I can't help thinking of how Kepler was cleverly able to provide many years of useful extended operation. So there are several questions around this topic:

  1. Could JWST be moved to an Earth-trailing orbit (or other useful orbit) without sacrificing too much precious remaining propellent?

  2. Is there a position in an Earth-trailing orbit that is far enough away that the Earth's heat is not a problem but close enough so that a reasonable data-rate link can be established?

  3. Is it possible to point the main antenna at Earth while the sunshield is still shading the telescope? Is there a low-gain antenna that might be useful?

  4. If Earth communications and stellar observations require different orientations, is there enough data storage capacity on board to make some useful obervations and then re-orient and dump the data to Earth?

  5. I gather that solar pressure on the sunshade can potentially be used to unload the momentum in the reaction wheels. Does mean that JWST could continue to be mostly oriented as desired with only an occasional excursion to a weird orientation to allow solar pressure to desaturate the reaction wheels?

Sorry for the multiple questions!

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  • $\begingroup$ With zero propellant I don't think its attitude control system can unload (angular) momentum from the reaction wheels. There might be a solution where torque from solar photon pressure on the sunshield is sufficient considering the solar photon pressure wizardry you elude to with K2 and also designed into GAIA. But if a new orbit is called for and can be gotten to with some very very low delta-v three-body wizardry, then perhaps it would be done while there was still... $\endgroup$
    – uhoh
    Dec 7, 2021 at 5:36
  • $\begingroup$ ...propellant for that solution & for a small amount of momentum unloading to maintain attitude. This question is wonderful, and even though it first seems to be in five parts, I'll bet that if there's a source out there where this has been addressed it will address all of it at once. Still, either a Restore-L type intrusive refueling mission (robotically splice into a fuel line) or a "strap-on" new propulsion module (there are several companies developing these) might be more desirable than letting it just drift off heliocentric sans juice $\endgroup$
    – uhoh
    Dec 7, 2021 at 5:39
  • $\begingroup$ For more on the fuel system and feasibility of a robotic, invasive refueling mission (not what you are asking about, but perhaps more likely to happen) see this answer to Is it possible to refuel the James Webb Space Telescope? $\endgroup$
    – uhoh
    Dec 7, 2021 at 5:45
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    $\begingroup$ @NgPh estimate a 'low' lifetime to tell congress it won't cost much, then use your healthy spacecraft as justification for extended missions that do cost more than they were initially willing to give you (e.g. Cassini, Curiosity, Juno, Ingenuity, Galileo, Magellan, etc...) $\endgroup$ Dec 7, 2021 at 12:56
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    $\begingroup$ @NgPh Spacecraft are sized with considerable margin. With regard to the JWST, ten years is a design goal but is not a requirement. The requirement is five years. If the spacecraft is operation for only four years there will be an extensive mission failure commission. Investigations will also occur if the spacecraft is only operational for nine years, but they will be much less extensive and the mission will still be deemed a success. Sizing a vehicle to only meet mission requirements is a recipe for mission failure because things always go bump in the night. $\endgroup$ Dec 7, 2021 at 14:05

1 Answer 1

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Could JWST be moved to an Earth-trailing orbit (or other useful orbit) without sacrificing too much precious remaining propellent?

The JWST will be moved to an Earth-trailing heliocentric orbit as an end of primary mission operation.

The amount of propellant in the JWST is sized to satisfying several key requirements:

  1. Perform mid course corrections, Sun-Earth L2 halo orbit insertion, and orbit maintenance maneuvers. These operations are performed with bipropellant thrusters the use hydrazine as fuel and dinitrogen tetroxide as oxidizer.
  2. Perform course attitude control and desaturate the momentum wheels. These operations are performed with monopropellant thrusters the use hydrazine only.
  3. Dispose of the spacecraft as an end of mission burn sequence so the spacecraft is not a threat to the Earth or to other spacecraft. I'm not sure whether this will use of the bipropellant or monopropellant thrusters.

Forgoing last item is not an option. Decommissioning as an end of the primary mission via disposal from the Sun-Earth L2 halo orbit has been part of the plan from day one. The JWST will not run out of propellant while on its Sun-Earth L2 halo orbit (and least not without an extensive mission failure analysis commission).

Regarding the subsequent questions, it is quite likely that the spacecraft will still have some hydrazine after the end of mission disposal burns as burning to depletion would represent a risk. There will be some margin, meaning that it is possible for the spacecraft to be repurposed as an Earth-trailing spacecraft after completing its primary mission.

However, thinking about that secondary mission right now is a bit premature. The spacecraft has not even launched yet. The focus right now and for at least the next six months are / will be laser focused on getting the spacecraft operational, and then it will be laser focused on keeping it operational.


References:

James Webb Space Telescope Program / Project Plan, JWST-PLAN-000633

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  • $\begingroup$ Yes, the question is definitely premature. I can't say I'm optimistic about JWST's deployment, but I sure hope it works! $\endgroup$
    – Roger Wood
    Dec 7, 2021 at 19:09
  • $\begingroup$ I can imagine there are several factors that might prevent JWST's use in a propellant-less mode in an Earth-trailing orbit. But I don't see you saying that in your answer. Does that mean there might be some sort of 'afterlife' for JWST? $\endgroup$
    – Roger Wood
    Dec 7, 2021 at 19:12
  • $\begingroup$ In 2007 in this draft of JWST-PLAN-000633 it says in 1.4.1.4 Operations Phase... 7.0 CONTINGENCY OPERATIONS; 7.3 Ground System Fault Detection and Recovery: "Upon completion of the JWST mission, the observatory will be decommissioned and, with the use of residual orbit station keeping fuel, will be “removed” from the L2 orbit and placed in a deep space coast orbit" In 2021 there may be a longer and more certain statement about this somewhere. Of course the answer will likely be the same $\endgroup$
    – uhoh
    Dec 20, 2021 at 2:54

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